Generally, an angio X-ray imaging apparatus includes an X-ray generating part, an X-ray detecting part, a supporting part which supports the X-ray generating part and the X-ray detecting part, a bed and a processor, for example. The supporting part is a C-arm or an Ω-arm, for example, and the supporting part moves such that images of a patient are obtained from several angles or positions.
As a detector of the detecting part, an X-ray film or an I.I. (Image Intensifier) is used, for example. In an imaging of the I.I., an X-ray tube of the X-ray generating part irradiates an X-ray to the patient, and the I.I. transfers the X-ray penetrated through the patient into an optical image. The optical image is changed to electric signals by an X-ray TV camera. The electric signals are converted by A/D converter and are displayed on a monitor as an X-ray image. In an imaging of the film, it is impossible to display the X-ray image in real time, however in the imaging of the I.I., a real time imaging can be performed. In addition, digital signals are obtained, several image processes can be performed. Recently, in stead of the I.I., a X-ray flat panel detector (referred to as a flat panel detector below) which has detection elements arranged in two dimension is developed.
It is required that an imaging part including the X-ray generating part and the X-ray detecting part speedily moves in a wide range in order to move the C-arm according to a flow of a contrast agent in a blood vessel.
In this case, in order to obtain clear image data, it is required to arrange the flat panel detector in a predetermined position close to a surface of a body of the patient surface, and when the flat panel detector contacts the body of the patient, a movement of the X-ray detecting part stops by using a contact type sensor as one method.
However, in this method, it is difficult to stop the X-ray detecting part quickly, and the patient may be contacted-to the X-ray detecting part.
Therefore, non-contact type capacitance sensor is used, recently.
In this method, the capacitance sensor is attached around the flat panel detector of the X-ray detecting part. Using information of the capacitance which changes according to a position of the patient, a distance of the patient's body surface and the flat panel detector is measured. Based on information of the measured distance, speed of the movement of the X-ray detecting part is slowed down gradually, and the X-ray detecting part stops at a predetermined position near the patient. In this method, it is possible to move the X-ray detecting part at high speed to a position close to the patient's body surface, and efficiency for diagnosis improves. And it is possible to obtain an image even if the flow of the blood is fast.
However, In the method using an above-mentioned capacitance sensor, even if the distance between the flat panel detector and the patient's body surface is constant, a value of the measured capacitance changes according to a shape, sex, age, degree of obesity, etc. of the patient. For this reason, it is difficult to stop the flat panel detector at a desired position, since the distance between the flat panel detector to be stopped and the patient body surface is different by each patient.
Regarding the problem caused by the above-mentioned patient's shape, first, by presuming the patient's surface based on change of the value of the capacitance according to the movement of the X-ray detecting part. Thereafter, the value of the capacitance to be measured is corrected based on the presumed patient's surface. Thereby, X-ray detecting part can be positioned at a desired position. The technique is disclosed in Japanese Patent Publication (Kokai) No. 2001-241910 (pp 4–7 and FIG. 1 to 9).
In this method, it is possible to set the X-ray detecting part at a appropriate position automatically by correcting the value of the capacitance measured even if the shape of the patient's body surface differs. However, since this method is complex, it is difficult to correct the value of the capacitance, constantly. Or, since the capacitance sensor is attached around the X-ray detecting part, an electromagnetic field formed is not be uniformed to the patient's body surface. When the surface of the patient is an odd-shaped, it is difficult to measure the distance between the patient and the X-ray detecting part correctly.
Further, in this method, it is also difficult to correct the value of the capacitance caused by error factors other than the patient's shape, the X-ray detecting part does not stop at a desired position.